Pecoraite
Chemical formula: Ni<sup>2+</sup><sub>3</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>
Hydrated nickel silicate of the serpentine group, forming characteristic, bright green coatings and earthy masses.
Description
## Characteristics Pecoraite is a mineral of the serpentine group, a hydrated nickel silicate. It most often occurs as very fine-grained, compact or earthy aggregates, as well as crusts and coatings. Due to its microcrystalline structure, well-formed crystals are not observed. Its most characteristic feature is an intense, apple-green or yellowish-green color, directly related to its high nickel content. ## Physical Properties This mineral is relatively soft, with a hardness of about 2.5 on the Mohs scale. The luster is described as earthy, dull, sometimes waxy in more compact aggregates. It is a translucent to opaque mineral. Its calculated density is approximately 3.57 g/cm³, which is a relatively high value for a mineral with an earthy habit. ## Colors and Varieties Pecoraite is monochromatic – its shades range from light green, through vivid, apple green, to yellowish-green. The intensity of the color depends on the degree of hydration and chemical purity. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral was described in 1969 by George T. Faust, Joseph J. Fahey, Brian H. Mason, and Edward J. Dwornik. Its name honors William Thomas Pecora (1913–1971), an American geologist and director of the United States Geological Survey (USGS), in recognition of his contributions to nickel deposit research. ## Uses Pecoraite has no industrial application. However, it is an object of interest for scientists and mineral collectors, especially specimens from unique localities.
Diagnostic features
## Identification The key diagnostic feature of pecoraite is its vivid, apple-green color combined with low hardness (it can be scratched with a fingernail) and an earthy or waxy form of occurrence. Its genesis, related to the weathering of ultramafic rocks or nickel meteorites, is also an important clue. ## Distinguishing from Similar Minerals Pecoraite is sometimes confused with garnierite, which, however, is not formally a mineral but a mixture of various hydrated nickel silicates. Annabergite (nickel bloom) has a similar color, but often forms small, acicular crystals and is an arsenate, not a silicate. Other minerals of the serpentine group rarely achieve such an intensely green coloration. ## Crystal Forms Pecoraite is a microcrystalline mineral. It forms earthy, compact, porous masses, as well as thin crusts and veins. Macroscopic crystals are not known.
Geological environment
## Genesis Pecoraite is a secondary mineral. It forms as a result of weathering and hydrothermal alteration of primary nickel-rich minerals, such as millerite, pentlandite, or awaruite. It forms in the oxidation zones of nickel sulfide deposits, in lateritic weathering crusts on ultramafic rocks (serpentinites, peridotites), and, which is its special feature, in weathered fragments of iron-nickel meteorites. ## Mineral Associations This mineral co-occurs with other weathering products. Depending on the environment of formation, it is accompanied by goethite, quartz, chalcedony, maghemite, as well as relics of primary minerals, such as millerite. In the case of occurrence in meteorites, its association includes kamacite, schreibersite, and other meteoritic minerals. ## Localities The type locality, from which pecoraite was first described, is the Wolf Creek meteorite crater in Western Australia, where it formed as a result of meteorite weathering. Other known localities include lateritic deposits in Moa Bay, Cuba, serpentinites near Ortitaia, Corsica (France), and some nickel deposits in Canada and Russia.
Rarity
Rare
Collector aspects
## Quality Criteria The most highly valued pecoraite specimens are characterized by an intense, saturated, apple-green color. The thickness and uniformity of the crust on the host rock (matrix) are also important. Specimens from the type locality (Wolf Creek Crater), where pecoraite occurs on meteorite fragments, are of particular value, representing a unique combination of extraterrestrial material and a secondary mineral. ## Popular Localities The most classic and sought-after by collectors are specimens from the Wolf Creek Crater in Australia. Material from this locality is considered exemplary for this mineral. Specimens from other localities, such as Corsica or Cuba, are also valued if they exhibit good quality of formation and color.
Care and storage
## Cleaning Pecoraite specimens are soft and prone to scratching. For dust removal, it is best to use a soft brush or compressed air from a safe distance. Avoid washing in water; if absolutely necessary, use a small amount of distilled water and immediately dry the specimen thoroughly. ## What to Avoid The mineral should be protected from mechanical damage, contact with harder minerals, and chemicals, especially strong acids. Due to its earthy, porous nature, contact with liquids that could penetrate its structure and cause discoloration or aggregate disintegration should be avoided. ## Storage It is recommended to store specimens in separate, sealed boxes or display cases to prevent abrasion, crumbling, and dust accumulation. Exposure should take place under stable conditions, away from sources of moisture and vibrations.